Abstract : A series of laboratory isothermal and continuous cooling heat-treatments were employed to develop and characterize the low temperature transformation products or microstructures which could be present in commercially produced large gun tube forgings. The tensile mechanical properties, hardness and Charpy V-notch impact transition curves were determined for each of the various microstructures produced. Of the three microstructures (martensite and two bainites) evaluated, tempered martensite produced the best combination of strength and toughness. Continuous cooling heat treating studies were used to demonstrate that a fully martensitic microstructure could be produced at the mid-radius of full size large gun tube forgings. Low yield strengths and impact energies were correlated with the tempered bainitic structures produced by transforming or quenching the gun steel forgings too slowly. (Author)